LSP: Investigate 'auto-imports from npm dependencies' hack for perf issues

[nayeemrmn]

https://github.com/nayeemrmn/deno/blob/9dbb99a83cb599028aef662b23e13faf2df15297/cli/lsp/tsc.rs#L4315-L4317

We use this to hide the fact that these paths are in a /node_modules/ folder because tsc wasn't giving auto-import suggestions from them. Did this have a knockback effect on performance? Could we only apply this to direct dependencies? How does node/tsc give these suggestions?

[nathanwhit]

Investigated the cause (didn't look at perf), the issue is this https://github.com/microsoft/TypeScript/blob/e9738050cf896a9c08d1d3d911f304513f80b76a/src/services/exportInfoMap.ts#L436-L453

Specifically

startsWith(getCanonicalFileName(fromPath), toNodeModulesParent)

it's checking if the import is in the top level node_modules. For an npm style node_modules layout, this works fine (example below from a project at /home/node-modules-completion:

DEBUG JS - TSC: isImportablePath 
fromPath="file:///home/node-modules-completion/main.ts"
toPath="file:///home/node-modules-completion/node_modules/preact/hooks/src/index.d.ts"
globalCachePath=undefined
toNodeModules="file:///home/node-modules-completion/node_modules"
toNodeModulesParent="file:///home/node-modules-completion"

startsWith(getCanonicalFileName(fromPath), toNodeModulesParent) === true

but with the pnpm style layout, when you fully resolve the symlinks this doesn't work:

DEBUG JS - TSC: isImportablePath
fromPath="file:///home/node-modules-completion/main.ts"
toPath="file:///home/node-modules-completion/node_modules/.deno/[email protected]/node_modules/preact/hooks/src/index.d.ts"
globalCachePath=undefined
toNodeModules= "file:///home/node-modules-completion/node_modules/.deno/[email protected]/node_modules"
toNodeModulesParent="file:///home/node-modules-completion/node_modules/.deno/[email protected]"

startsWith(getCanonicalFileName(fromPath), toNodeModulesParent) === false

The reason the current workaround works is that we effectively force toNodeModules=undefined and toNodeModulesParent=undefined, which makes isImportablePath just always return true.